Interferon-gamma elevates nicotinamide N-methyltransferase activity and nicotinamide level in human glioma cells

Nicotinamide N-methyltransferase (NNMT) catalyzes the N-methylation of nicotinamide. NNMT is strongly expressed in tumor cells and an increase in NNMT activity may reduce cellular nicotinamide level and thereby promote cell survival in the cells. However, there has been no report of a relationship between NNMT activity and nicotinamide level in tumor cells. We report herein that human glioma cells produce relatively large amounts of NNMT and that when these cells are cultured in the presence of interferon-gamma (IFN-gamma) their 1-methylnicotinamide levels increase. To clarify the mechanisms by which IFN-gamma increases 1-methylnicotinamide levels in these cells, we measured NNMT activity and the levels of NNMT expression, nicotinamide and nicotinamide adenine dinucleotide (NAD(+)) in the presence and absence of IFN-gamma. We also examined whether addition of exogenous 1-methylnicotinamide directly affects cell viability and/or the cellular levels of 1-methylnicotinamide, nicotinamide and NAD(+). While addition of 1-methylnicotinamide increased the total amount of cellular 1-methylnicotinamide present, it did not affect nicotinamide or NAD(+) levels, or cell viability. Conversely, IFN-gamma significantly increased NNMT activity and the nicotinamide cellular concentration, while leaving NNMT expression and the NAD(+) cellular concentration unchanged. Therefore, the increase in the 1-methylnicotinamide level found when IFN-gamma is present in culture may be a consequence of increases in both the nicotinamide concentration and NNMT activity, whereas, 1-methylnicotinamide did not influence nicotinamide levels, NAD(+) levels, or cell viability per se. These results suggest that an increase in NNMT activity does not always reduce cellular nicotinamide concentration in tumor cells.